Thanks for the additional information. I am very worried about your kitty because he has not been feeling well for several days, and because he will not eat for you today. I think he will need to see an emergency vet tonight. It is very dangerous for a kitty to go without eating and the longer your cat goes without medical attention, the more dehydrated and decompensated he will become. I am afraid that if has to wait until Monday to see his vet he will be beyond help. My concern may seen extreme, but cats instinctively hide illness, and it is possible that your kitty was sick for longer than you are aware. It is frightening when they show obvious signs of illness so suddenly.
As for the causes of the lethargy and decreased appetite, there are quite a few, and it is difficult to narrow it down exactly for you with such vague symptoms. But, cats are considered to be geriatric at 8 years old, and are prone to illness as they age. The 3 most common ailments known to afflict elderly cats are diabetes, kidney disease and hyperthyroidism. The symptoms include vomiting, diarrhea, increased water consumption, increased urination, dehydration, weakness, weight loss, muscle wasting, poor hair coat, changes in behavior, changes in appetite, and increased vocalizations.
I am particularly concerned about the swallowing difficulties, and this can be due to dehydration, one of the illnesses above (hyperthyroidism has been the illness most closely associated with loss of voice or voice changes, so it may be manifesting as trouble swallowing), or to cancer.
Your kitty is going to become very sick in a very short period of time. He really needs to see an emergency vet tonight to try and figure out what is going on, and for treatment to be started. Maybe you'll luck out and he will not need to be hospitalized, but an exam and some diagnostics (blood panel, urinalysis and radiographs) will give alot more information. Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Good luck with your kitty!